1. Field of the Invention
The present invention relates to bandpass filters used in, for example, communication equipment for a band from microwaves to millimeter waves and, more particularly, to a bandpass filter configured to suppress spurious signals that occur depending on the positional relationship between ground electrodes and a resonator electrode.
2. Description of the Related Art
Various dual-mode bandpass filters have been used as bandpass filters for use in high-frequency bands.
For example, a dual-mode bandpass filter including a resonator electrode having an aperture is disclosed in Japanese Unexamined Patent Application Publication No. 2001-237610. FIG. 17A is a cross-sectional front view and FIG. 17B is a plan view, schematically showing a dual-mode bandpass filter 101 disclosed in this publication. The dual-mode bandpass filter 101 includes a dielectric substrate 102. A resonator electrode 103 is provided at an intermediate height in the dielectric substrate 102. The resonator electrode 103 includes an aperture 103a (an area where no electrode is formed in the resonator electrode 103). The resonator electrode 103 includes a plurality of non-degenerate resonant modes. The aperture 103a is arranged to couple the resonant modes to each other to define the dual-mode bandpass filter. Ground electrodes 104 and 105 are provided beneath the top surface and on the bottom surface of the dielectric substrate 102, respectively, so as to oppose the resonator electrode 103. Referring to FIG. 17B, input-output coupling electrodes 106 and 107 are connected to the resonator electrode 103. The input-output coupling electrodes 106 and 107 extend outside the resonator electrode 103, although not shown in FIG. 17A, and are electrically connected to the corresponding input-output terminal electrodes (not shown).
Usually, in a bandpass filter, such as the dual-mode bandpass filter 101, including the ground electrodes provided over and under the resonator electrode with dielectric layers of the dielectric substrate disposed therebetween, ground electrodes are also provided on side surfaces of the dielectric substrate 102. Accordingly, the ground electrodes define a waveguide, that is, the resonator electrode 103 is provided in a waveguide. With such a structure, resonances occur depending only on the shape of the waveguide. Consequently, the structure, similar to a waveguide, including the ground electrodes is larger than the resonator electrode 103.
The fundamental resonances caused by the ground electrodes occur at frequencies lower than the resonant frequency of the resonator electrode 103, and higher-mode resonances sequentially occur at overlapping positions with resonant modes caused by the resonator electrode 103. Such resonances caused by the ground electrodes produce undesired spurious signals in the dual-mode bandpass filter 101, such that it is impossible to achieve good transmission characteristics.